Previous and current research on piezoresistivity of polymer composites filled with carbon nanostructures is reviewed. The review covers the use of the coupled electro‐mechanical response of these materials to self‐sense their strain and damage during mechanical loading. The mechanisms yielding changes in electrical resistance upon mechanical loading in polymer composites filled with carbon nanostructures are first discussed. Published knowledge is then summarized, starting with framework literature on carbon black and graphite and then moving to more recent research on carbon nanotubes, exfoliated graphite, and few‐layer graphene sheets. Piezoresistive studies of polymer nanocomposites with aligned carbon fillers are also reviewed. It is aimed that this review contributes in collecting, organizing, and summarizing the knowledge, foundations, and state of the art on the piezoresistive response of polymer composites filled with different carbon allotropes, providing new perspectives and advancing towards the fast development of smart self‐sensing carbon filled nanocomposites.
Carbon nanotube yarns (CNTYs) are hierarchical fibers with outstanding electrical properties, and understating the temperature‐dependence of their electrical resistance (thermoresistivity) is essential for sensing applications and development of self‐sensing polymer composites. The cyclic thermoresistive response of individual CNTYs and the effect of embedding the yarn into a polymer are experimentally investigated herein. The effect of confining the CNTY by a thermosetting polymer is addressed by studying the thermoresistive response of CNTY/vinyl ester single‐fiber composites. Heating–cooling cycles ranging from 25 (room temperature [RT]) to 100 °C and 25 to −30 °C are applied to individual CNTYs and to CNTYs embedded into a vinyl ester polymer, while their electrical resistance is simultaneously recorded. Both the CNTY and its single‐fiber composite show a negative dependence of electrical resistance with temperature. For both temperature ranges (above and below RT), the average temperature coefficient of resistance found for individual CNTYs is ≈−9.5 × 10−4 K−1, and its magnitude decreases about ≈30% when the yarn is embedded into the vinyl ester polymer. The hysteresis rendered by the different heating and cooling pathways is small for individual CNTYs, and largely increases when the CNTY is embedded into the polymer.
Polyimide films are currently of great interest for the development of flexible electronics and sensors. In order to ensure a proper integration with other materials and PI itself, some sort of surface modification is required. In this work, microwave oxygen plasma, reactive ion etching oxygen plasma, combination of KOH and HCl solutions, and polyethylenimine solution were used as surface treatments of PI films. Treatments were compared to find the best method to promote the adhesion between two polyimide films. The first selection of the treatment conditions for each method was based on changes in the contact angle with deionized water. Afterward, further qualitative (scratch test) and a quantitative adhesion assessment (peel test) were performed. Both scratch test and peel strength indicated that oxygen plasma treatment using reactive ion etching equipment is the most promising approach for promoting the adhesion between polyimide films.
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